1. Field of the Invention
The present invention relates generally to a semiconductor device, and the invention is applicable to, e.g. a transistor in which the mobility of electrons or holes is varied by applying a stress to a semiconductor substrate.
2. Description of the Related Art
In the prior art, as one of active devices that constitute an LSI (large-scale integration), there is known an insulated-gate field-effect transistor (hereinafter referred to as “transistor”) that is represented by a MOS (metal oxide semiconductor) transistor or a MIS (metal insulator semiconductor) transistor. With development in fine patterning of the transistors, the number of transistors in the LSI has become enormous. Being in direct proportion to the integration level, the heat emitted from the transistors becomes more and more intensive as the number of the transistors increases. As a result, the degree of lattice vibration of the crystal lattice of, e.g. silicon that forms the transistor becomes higher and the corresponding thermal disturbance becomes more and more severe, decreasing the mobility of electrons or holes (carriers).
To solve this problem, a semiconductor device has been proposed, wherein an insulating material that imparts a stress is buried in suitable places around the transistors and, as a result, a desired stress is applied to the channel region of the transistor, thus improving the mobility of electrons or holes (see Jpn. Pat. Appln. KOKAI Publication No. 2004-63591, for instance).
In this structure, however, the stress that can be imparted by the insulating material is invariable in relation to a temperature rise of the semiconductor substrate, etc. Thus, when the temperature of the LSI rises from room temperature to high temperatures (e.g. about 200° C.), the effect of the stress becomes insufficient due to the intensified thermal disturbance of silicon, etc. As a result, the mobility of electrons or holes decreases.
Consequently, there is such a tendency that the mobility of electrons or holes (carriers) becomes lower as the temperature of the LSI rises.